Archive-name: LANs/token-ring-faq
Posting-Frequency: monthly
Last-modified: 1999/07/08
Version: 990708
URL: http://www.NetworkUptime.com/faqs/token-ring
Copyright: (c) 1999 James Messer
Maintainer: James Messer
comp.dcom.lans.token-ring Frequently Asked Questions
----------------------------------------------------
This document is provided as is without any express or implied
warranties. While every effort has been taken to ensure the accuracy of
the information contained in this article, the authors assume no
responsibility for errors or omissions, or for damages resulting from the
use of the information contained herein. The contents of this article
reflect my opinions only and not necessarily those of my employer.
FAQ Table of Contents
---------------------
1.0 FAQ Administration
[1.1] What is this FAQ?
[1.2] Who maintains this FAQ?
[1.3] Where can this FAQ be found?
[1.4] Who provides information to the FAQ?
[1.5] Can I use this FAQ on my web page?
[1.6] Copyright Information
2.0 Introduction to Token Ring
[2.1] What is token ring?
[2.2] How do Ethernet and token ring networks compare?
[2.3] Where are the IEEE specifications?
3.0 General Token Ring Information
[3.1] How does token ring work?
[3.2] What is used to convert between Ethernet and Token Ring?
4.0 Token Ring Physical Layer
[4.1] What physical devices are required for a token ring network?
[4.2] What types of cables are used for token ring?
[4.3] What pin assignments are used in token ring cabling?
[4.4] What is the difference between a MAU, a CAU, and a LAM?
[4.5] Can two token ring stations be directly attached?
[4.6] What is the maximum distance between token ring stations?
[4.7] What is the formula for computing adjusted ring length (ARL)?
[4.8] Why is ring length important?
[4.9] At what speeds does token ring run?
[4.10] How many stations are supported by a single token ring network?
[4.11] What is High Speed Token Ring?
5.0 Token Ring Data Link Layer
[5.1] What is a token?
[5.2] What are MAC frames?
[5.3] What are LLC frames?
[5.4] What are Locally Administered Addresses (LAAs)?
[5.5] What are functional addresses?
[5.6] What is an Active Monitor and Standby Monitor?
[5.7] What is early token release?
[5.8] What is transparent bridging?
[5.9] What is spanning tree bridging?
[5.10] What is source route bridging?
[5.11] What is token ring switching?
[5.12] What is the process for inserting into a ring?
[5.13] How do you troubleshoot the insertion process?
6.0 Token Ring Errors and Troubleshooting
[6.1] What are isolating and non-isolating errors?
[6.2] What is the claim process?
[6.3] What is a beacon frame?
[6.4] What is promiscuous mode?
[6.5] What software is available to monitor a token ring network?
7.0 Other Information
[7.1] What token ring books are available?
[7.2] What certifications are available regarding token ring networks?
[7.3] What companies make token ring adapter cards and MAUs?
1.0 FAQ Administration
[1.1] What is this FAQ?
This FAQ will attempt to explain and decipher the intricacies of
token ring networking and answer some of the most common questions
relating to token ring networks. Although it contains technical
information, this FAQ is best used as an introduction to token ring
networking. See section [7.1] for token ring book and publication
information.
[1.2] Who maintains this FAQ?
This FAQ is maintained by James Messer .
Questions, comments, corrections, and contributions are encouraged!
[1.3] Where can this FAQ be found?
This FAQ will be posted to the comp.dcom.lans.token-ring newsgroup
on the first of each month. An archive of the FAQ can be found at:
ftp://rtfm.mit.edu/pub/faqs/LANs/token-ring-faq
A HTTP version of this FAQ can be found at:
http://www.NetworkUptime.com/faqs/token-ring
[1.4] Who provides information to the FAQ?
In many cases, the FAQ questions and answers are summarized from the
comp.dcom.lans.token-ring newsgroup. Other submissions to the FAQ
were contributed by:
Lawrence L. Baldwin
Kris Carlier
David Holbrook
Neil Jarvis
Bernie Keenan
John Kristoff
Mark R. Kuijper
Richard F. Masoner
Michele Mastroianni
Send any corrections or FAQ additions to James@NetworkUptime.com.
Our thanks to all who have provided information to the FAQ! Keep
those submissions coming!
[1.5] Can I use this FAQ on my web page?
Since this FAQ changes almost daily, a copy of the FAQ on your web
page would be out of date in a very short time. Please don't do
this! A more appropriate method would be to set a hyperlink to the
URL found in the secondary header of this FAQ. Please send an e-mail
to James@NetworkUptime.com if you plan on adding a link to this FAQ
to your web page. I reserve the right to restrict the use of this
FAQ.
[1.6] Copyright Information
Copyright (c) 1999 by James Messer, all rights reserved.
This FAQ may be posted to any USENET newsgroup, on-line service, or
BBS as long as it is posted in its entirety, includes this copyright
statement, and includes written permission from
James@NetworkUptime.com.
2.0 Introduction to Token Ring
[2.1] What is token ring?
Token ring is the IEEE 802.5 standard that connects computers
together in a closed ring. Devices on the ring cannot transmit data
until permission is received from the network in the form of an
electronic 'token'.
[2.2] How do Ethernet and token ring networks compare?
Token Ring is single access, meaning there is only one token. Thus,
at any given time only one station is able to use the LAN. Ethernet
is a shared access medium, where all stations have equal access to
the network at the same time.
[2.3] Where are the IEEE specifications?
The IEEE specifications can be purchased from the IEEE at:
http://standards.ieee.org/catalog/IEEE802.3.html
Information on all IEEE standards can be found at:
http://www.ieee.com
For more information on the 802.5 standards, see The official IEEE
802.5 web site at:
http://p8025.york.microvitec.co.uk/
3.0 General Token Ring Information
[3.1] How does token ring work?
A token ring network uses a special frame called a token that
rotates around the ring when no stations are actively sending
information. When a station wants to transmit on the ring, it must
capture this token frame. The owner of the token is the only station
that can transmit on the ring, unlike the Ethernet topology where
any station can transmit at any time. Once a station captures the
token, it changes the token into a frame format so data can be sent.
As the data traverses the ring, it passes through each station on
the way to the destination station. Each station receives the frame
and regenerates and repeats the frame onto the ring. As each station
repeats the frame, it performs error checks on the information
within the frame. If an error is found, a special bit in the frame
called the Error Detection bit is set so other stations will not
report the same error.
Once the data arrives at the destination station, the frame is
copied to the destination's token ring card buffer memory. The
destination station repeats the frame onto the ring, changing two
series of bits on the frame. These bits, called the Address
Recognized Indicator (ARI) and the Frame Copied Indicator (FCI),
determines if the destination station had seen the frame and has had
ample buffer space available to copy the frame into memory. If the
frame is not copied into memory, it is the responsibility of the
sending station to retransmit the frame.
The frame continues around the ring, arriving back at the source
station who recognizes the sending address as it's own. The frame is
then stripped from the ring, and the source station sends a free
token downstream.
[3.2] What is used to convert between Ethernet and Token Ring?
There is no 'converter' that allows an Ethernet network and Token
Ring network to communicate between each other. A conversion process
must occur between the two topologies, since they both use different
signaling types, frame structures, and frame sizes.
There are two methods to accomplish this 'conversion'; bridging, and
routing.
Bridging
--------
Bridging is a method of communicating between devices at OSI layer
2, the data link layer. A bridge connects two networks together and
acts as a traffic director. If traffic is destined to the other
network, the bridge allows the traffic to pass. If the traffic is
local to a single network, the bridge does not pass the traffic
unnecessarily to the other connected network.
The bridge makes this determination based on the Media Access
Control (MAC) address of the workstations on the network. The bridge
keeps an updated list of everyone active on the network, and uses
this list to direct traffic from one network to another.
This method of operation makes the network appear as a single
logical network, since the only separation of traffic from one
network to another is done at the MAC address level.
There are many bridge manufacturers and bridge types on the market.
The newest version of this bridging technology is called a DLC
Switch or LAN Switch. These switches have a much higher port density
than the older two or three port bridges, allowing for much more
flexibility and network segmentation.
Routing
-------
The second method of 'converting' from Ethernet to Token Ring is
called routing. Routing occurs at OSI layer 3, and separates
physical networks into separate logical networks. This
differentiates routing from bridging, since bridging maintains a
single logical network.
In a routed network, the sending workstation determines if outgoing
traffic is local or remote. If the traffic belongs to another
network, the originating station sends the frame directly to the
router for further processing.
Upon receiving the frame from the source workstation, the router
examines the frame for the destination address. The router maintains
a routing table which is used to determine the final destination of
the data packet through the router.
Routing is the most common method of connecting Ethernet networks to
Token Ring networks in most organizations. Most network operating
systems have routing capabilities built into the servers. By placing
a token ring and Ethernet card into a Novell NetWare 3.x/4.x or
Windows NT v4.x server, the two topologies can communicate between
each other.
One caveat; some protocols are not routeable. A good example is
Microsoft's NetBEUI, which has no OSI layer 3 network address and
therefore cannot be routed. Protocols which cannot be routed must be
bridged between physical networks.
4.0 Token Ring Physical Layer
[4.1] What physical devices are required for a token ring network?
Token ring connectivity requires three separate physical entities; a
Multistation Access Unit (MAU), a token ring lobe cable, and a token
ring adapter card.
A Multistation Access Unit (MAU or MSAU) is a hub-like device that
connects to all token ring stations. Although the token ring
stations are attached to the MAU in a physical star configuration, a
true ring is maintained inside the MAU.
Unlike an Ethernet hub, a MAU consists of physical or electronic
relays which keep each station in a loopback state until a voltage
is sent from the station to the MAU. Since this voltage does not
affect data communications, it is referred to as a 'phantom'
voltage. Once this phantom voltage is received by the MAU, a relay
is activated that inserts the token ring station onto the ring.
MAUs are connected together with Ring In/Ring Out (RI/RO) cables. To
maintain a true ring, both the RI and the RO ports must be connected
from one MAU to the other.
A token ring lobe cable connects the token ring station to the MAU.
This cable communicates over four wires; two for transmit and two
for receive. The cable can be Shielded Twisted Pair (STP) or
Unshielded Twisted Pair (UTP).
A token ring adapter card is the physical interface that a station
uses to connect to a token ring network. There are token ring
adapter cards for almost every computer bus type.
[4.2] What types of cables are used for token ring?
There are three major physical token ring cabling systems; Shielded
Twisted Pair (STP), Unshielded Twisted Pair (UTP), and optic fiber.
[4.3] What pin assignments are used in token ring cabling?
An IBM-type Data Connector or Universal Data Connector (IDC or UDC),
is a hermaphroditic connector (neither male nor female). These
connectors attach to each other without having a specified male or
female connector type on each end. These connectors are commonly
found on IBM Type 1 cabling, a two-pair shielded cable.
The UDC connector has the following cabling requirements:
Red - Receive +
Green - Receive -
Orange - Transmit +
Black - Transmit -
A DB-9 connector uses four wires (two pairs) for token ring
networking:
Pin 1 - Red - Receive +
Pin 5 - Black - Transmit -
Pin 6 - Green - Receive -
Pin 9 - Orange - Transmit +
A RJ-45 connector is an eight wire twisted pair cable:
Pin 3 - Blue/White - Transmit -
Pin 4 - White/Orange - Receive +
Pin 5 - Orange/White - Receive -
Pin 6 - White/Blue - Transmit +
RJ-11 connectors are rarely used:
Pin 2 - Blue/White - Transmit -
Pin 3 - White/Orange - Receive +
Pin 4 - Orange/White - Receive -
Pin 5 - White/Blue - Transmit +
[4.4] What is the difference between a MAU, a CAU, and a LAM?
A MAU is a 8228 Multistation Access Unit. This unit provides eight
workstation connectors and 2 MAU ports (also called Ring In/Ring Out
ports).
A CAU is a 8230 Controlled Access Unit (Basically a MAU with
intelligence). A CAU supports up to four LAMs. The Ring In/Ring Out
ports of a CAU are copper, but can replaced with fiber connectors.
A LAM is a Lobe Attachment Module for the 8230. Each LAM supports 20
workstations.
[4.5] Can two token ring stations be directly attached?
Unlike Ethernet stations, token ring stations _cannot_ be directly
attached with a cross-over cable. Because of the process required
for inserting into a ring, a loopback process must complete and
phantom voltage must exist on a wire for a relay to open. A MAU must
be used to directly connect two workstations.
However, some token ring switches allow a station to directly
connect to a _switch_. This Direct Token Ring (DTR) connection is a
non-standard method of connecting a switch and a workstation onto a
single ring. This non-standard DTR connectivity does _not_ allow for
two workstations to be directly connected.
[4.6] What is the maximum distance between a MAU and a token ring
station, or between two token ring stations?
In token ring networking, distance requirements are different from
vendor to vendor. In general terms, the recommended standard
distance between stations for Type 1 cabling is approximately 300
meters, and the recommended standard distance between stations for
UTP cabling is about 150 meters.
Token ring distances are computed as the distance between repeaters.
IN a token ring network, each Network Interface Card (NIC) is a
repeater. Therefore, the length between stations cannot exceed the
cable lengths listed above.
Some manufacturers use 'active' MAUs which can regenerate the token
ring signal and act as a repeater. In these cases, the distances
between the token ring workstations and the MAUs can be much larger
than many 'passive' MAUs. Many active MAUs have other network
management features such as SNMP capabilities and auto-station
removal for stations inserting at the incorrect speeds.
[4.7] What is the formula for computing adjusted ring length (ARL)?
The adjusted ring length of a token ring network is the sum of all
cable lengths between wiring closets, minus the shortest cable
between wiring closets. The ARL is used to determine the total
length of the ring, and the maximum lobe distances (see section
[4.8]).
This calculation determines the ring length if part of the ring is
removed for troubleshooting. When a cable is removed from a Ring
In/Ring Out port, the loop-back creates a much larger ring than
normal. The ARL calculation defines the largest ring size that can
occur, based on the shortest cable between wiring closets.
[4.8] Why is ring length important?
The design of any network is dependent on limits. In token ring
networks, ring length is a large factor in the physical design of an
error-free network. If the ring is too long, timing and attenuation
issues can create physical-layer errors, disrupting communication
over the entire ring.
In the design of a token ring network, total ring length dictates
the maximum length of cable between the workstation and the MAU.
This value, called the lobe length, can be computed with a series of
tables. These tables are computed for passive MAU networks. Active
MAUs provide capabilities that deviate greatly from the values in
these tables. Consult the manufacturer of the active MAUs for values
that are appropriate for that equipment.
[4.9] At what speeds does token ring run?
Token ring runs at speeds of 4 megabits per second (500,000 bytes
per second) and 16 megabits per second (2,000,000 bytes per second).
Some token ring switches support a non-standard referred to as
Direct Token Ring (DTR), or full-duplex token ring. This allows for
16 megabit speeds in the sending and receiving directions
simultaneously, for a maximum of 32 megabits per second (4,000,000
bytes per second).
[4.10] How many stations are supported by a single token ring network?
Again, this number is dependent on the token ring equipment that is
used in the network. Current standards list a maximum of 72 stations
on a UTP ring, and approximately 250 to 260 on a Type 1 network.
[4.11] What is High Speed Token Ring?
High Speed Token Ring, or HSTR, is a new token ring standard that
promises to push token ring speeds to 100 Mbps and 1 Gbps. The High
Speed Token Ring Alliance consists of 3Com, Bay Networks, IBM,
Madge, Olicom, UNH Interoperability Lab, and Xylan.
The first HSTR specification will allow for 100 Mbps token ring
speeds over both Type 1 and UTP copper cabling. Further
specifications will tackle 100 Mbps token ring over fiber. These
standards are due for completion in June or July of 1998. Another
HSTR specification will allow for 1 Gbps HSTR over fiber, and this
standard is due to be completed at the end of 1998.
For more information on HSTR, see:
http://www.hstra.com
5.0 Token Ring Data Link Layer
[5.1] What is a token?
A token frame is a three byte frame that takes this format:
+--------+--------+--------+
| SDEL | AC | EDEL |
| 1 byte | 1 byte | 1 byte |
+--------+--------+--------+
The Starting Delimiter (SDEL) byte is coded as JK0JK000, where the J
and K bits are intentional Manchester encoding violations. These
intentional violations delineate the token from normal traffic data.
J is the encoding violation of a 1, and K is the encoding violation
of a 0.
The Access Control (AC) byte is coded as PPPTMRRR. The priority bits
(PPP) provide eight levels of priority (000 through 111). The token
indicator bit (T) of 0 determines that the following information is
a token, a 1 designates the following information is a frame. The
Monitor bit (M) is used to prevent frames from constantly circling
the ring. The Priority Reservations bits (RRR) provide token
reservation to ring stations.
The Ending Delimiter (EDEL) byte is coded as JK1JK1IE, where the J
and K bits are encoding violations and the I and E bits are the
intermediate frame and error detection bits, respectively. The
intermediate bit is set to 1 if there are more frames to transmit in
this set. The error detection bit is set to 1 by a station that
recognizes a CRC error in the frame so other stations downstream do
not report the same error.
[5.2] What are MAC frames?
A Media Access Control (MAC) frame is used for management of the
token ring network. MAC frames do not traverse bridges or routers,
since they carry ring management information for a single specific
ring.
The MAC frame has this format:
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| SD | AC | FC | DA | SA |Data | FCS | ED | FS |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Size 1 1 1 6 6 >=0 4 1 2
in bytes
Starting Delimiter (SD) - See section [5.1].
Access Control (AC) - See section [5.1].
Frame Control (FC) - The frame control field consists of eight bits,
coded as TT00AAAA. The Frame Type bits (T) indicate the frame type.
Bits 2 and 3 are reserved, and are always zero. Bits four through
eight are Attention Codes which provide the token ring adapter of
incoming MAC information that can be copied to a special Express
Buffer in the token ring adapter.
Destination Address (DA) - The Destination Address specifies which
station is to receive the frame. The Destination Address can be sent
to a specific station, or a group of stations.
Source Address (SA) - The Source Address is the MAC address of the
sending station.
Data - A MAC frame data field contains token ring management
information, and a non-MAC (LLC) data field contains user data.
Frame Check Sequence (FCS) - A 32 bit Cyclical Redundancy Check
(CRC) is performed on the frame data to provide an integrity check
of the frame data. As each station copies the frame, the CRC is
computed and compared with the value in the FCS frame to verify that
the frame data is correct.
Ending Delimiter (ED) - See section [5.1].
Frame Status (FS) - The Frame Status field provides information for
the sending station regarding the status of the frame as it
circulates the ring. The Frame Status field is coded as AF00AF00.
The bits of the Frame Status field are duplicated, since this field
does not fall under the CRC checking of the Frame Check Sequence
bytes. The Address Recognized Indicator (ARI) is set to 1 by the
destination station if the destination station recognizes the frame.
The Frame Copied Indicator (FCI) is set to 1 if the destination
station was able to copy the frame into the local adapter buffer
memory.
[5.3] What are LLC frames?
A Logical Link Control (LLC) frame is used to transfer data between
stations.
LLC frames have the same frame structure as MAC frames, except frame
type bits of 01 are used in the Frame Control (FC) byte.
[5.4] What are Locally Administered Addresses (LAAs)?
Token ring addresses are either locally administered or universally
administered. Locally administered addresses are assigned by a local
manager and universally administered addresses are assigned by a
standards organization. Locally administered addresses are
designated by bit one set to 1 in byte zero of the destination
address field.
[5.5] What are functional addresses?
Functional addresses are assigned by the token ring specification to
allow for communication to functional devices. Some devices include:
Device Functional Address
------ ------------------
Active Monitor C0 00 00 00 00 01
Ring Parameter Server C0 00 00 00 00 02
Ring Error Monitor C0 00 00 00 00 08
Configuration Report Server C0 00 00 00 00 10
Source Route Bridge C0 00 00 00 01 00
[5.6] What is an Active Monitor and Standby Monitor?
Devices are either active monitors or standby monitors. There can
only be a single active monitor on a physical token ring. Any
station on the ring can assume the role of Active Monitor. All other
stations on the ring are standby monitors.
The Active Monitor provides many functions on a token ring network:
* The Active Monitor is responsible for master clocking on the token
ring network and the lower level management of the token ring
network.
* The Active Monitor inserts a 24-bit propagation delay to prevent
the end of a frame from wrapping onto the beginning of the frame.
* The Active Monitor confirms that a data frame or good token is
received every 10 milliseconds. This timer sets the maximum possible
frame size on a token ring network to 4048 bytes on a 4 megabit
ring, and 17,997 bytes on a 16 megabit ring.
* The Active Monitor removes circulating frames from the ring. As a
frame passes the Active Monitor, a special bit called a monitor
count bit is set. If the monitor count bit is set, the Active
Monitor assumes the original sender of the frame was unable to
remove the frame from the ring. The Active Monitor purges this
frame, and sends a Token Error Soft Error to the Ring Error Monitor.
If the Active Monitor is removed from the ring or no longer performs
the Active Monitor functions, one of the Standby Monitors on the
ring will take over as Active Monitor.
[5.7] What is early token release?
In normal token ring operation, a station sending information holds
the token until the sending data circles the entire ring. After the
sending station strips the data from the ring, it then issues a free
token.
With Early Token Release (ETR), a token is released immediately
after the sending station transmits its frame. This allows for
improved performance, since there is no delay in the downstream
neighbor waiting for the token.
ETR is only available on 16 megabit rings. Stations running ETR can
coexist with stations not running ETR.
[5.8] What is transparent bridging?
Transparent bridging is a method to connect two similar network
segments to each other at the datalink layer. It is done in a way
that is transparent to end stations, hence end-stations do not
participate in the bridging algorithm.
Transparent bridges are sometimes called (self) learning bridges.
When they are turned on and receive data packets from a network
segment they
1) learn the relation between MAC address and segment/port, and
2) forward the packet to all (!) other segments/ports.
The first step in this process is essential to the "learning" aspect
of the bridge. After some time the bridge has learned that a
particular MAC address, say MACa, is on a particular segment/port,
say PORT1. When it receives a packet destined for the MAC address
MACa (from any port not being PORT1) it will no longer forward the
packet to all ports (step 2). It knows that MACa is associated with
PORT1 and will only forward the packet to PORT1.
Please note that transparent bridging is most often used in a
Ethernet environment. In a token-ring environment it can be used but
is not common. In a token-ring environment source route bridging is
most often used.
[5.9] What is the spanning tree protocol?
Spanning tree is a protocol defined in IEEE 802.1q to prevent
bridges from creating network loops. Using the spanning tree
protocol, bridges communicate to each other and disable certain
ports/segments to prevent looping of packets.
Many implementations of the spanning tree protocol are configured so
an alternate path is available to network traffic, should the
original path become disabled.
[5.10] What is source route bridging?
Source route bridging is a method to connect two similar network
segments to each other at the datalink layer. It is done in a
"distributed way" where end-stations participate in the bridging
algorithm, thus the name _source_ routing. (as compared to
transparent bridging, refer to 5.9]).
In a source-route bridging environment a source end-station will
sent out a "route explorer" frame (broadcast) to find out the route
to the destination end-station. Source route bridges will forward
these frames to all segments/ports. The source route bridge will add
route information (the segment the packet came from) to the frame
prior to forwarding it. This route information is called the Routing
Information Field (RIF).
Eventually, the route explorer frame reaches the destination
end-station INCLUDING THE COMPLETE ROUTE (via the RIF) the packet
took. The destination end-station then uses this RIF to reply to the
source end-station directly (i.e. no broadcast). Please note that
